Preheat the oven to 375°F. Place the potatoes and celeriac in a medium saucepan and cover them with water. Add the bay leaf, 1 teaspoon of the salt, the garlic, and milk, bring to a boil, and then reduce heat. Simmer for 1 minute and then drain. Remove the bay leaf and discard it.
Coat a 10-inch oval gratin or a casserole dish with butter and add half of the potatoes and celeriac, making sure they are evenly distributed.
Sprinkle the figs, some salt, half of the Gruyère, and the pinch of nutmeg on top. Repeat with the remaining potatoes and celeriac and Gruyère and then pour the cream over the top.
Cover the dish with aluminum foil, place it in the oven, and bake for 20 minutes. Remove the aluminum foil and bake until the top is browned and most of the liquid has been cooked off, about 15 minutes.
Remove the dish from the oven and let rest for 15 minutes before serving.
